Hello all. I'm having trouble getting my 92 3.0 to pass smog. It passed 2 years ago with flying colors after a complete engine rebuild, but doesn't seem to want to cooperate this year. These are the stats:

First try:
15mph
HC-127 max 314gp 127 meas- pass
CO-.55 max 2.08gp 0.08 meas- pass
NO- 721 max 1822gp 683 meas- pass

25mph
HC-75 max 264gp 40 meas- pass
CO- 0.69 max 2.29gp 0.04 meas- pass
NO- 623 max 1622gp 2133 meas- fail, gross polluter

I did a full tune up, and threw on a new catalytic converter to boot. A fair chunk of change, but that's what the mechanic recommended. Feeling confident, I went back for a retest, and promptly failed again. These are my numbers after the new Cat.

15mh
HC- 127 max 314gp 37 meas- pass
CO- .55 max 2.08gp 0.01 meas- pass
NO- 721 max 1822gp 1066 meas- FAIL

25mph
HC- 75 max 264gp 18 meas- Pass
CO- .69 max 2.29gp 0.01 meas- Pass
NO- 623 max 1622gp 1704 meas- FAIL!

I'm a little confused as to why my 15mph numbers went up after the new Cat, and wondering what I should do next. My first thoughts are towards a new upstream O2 sensor and new EGR valve, but I would love any advice as to what I might be missing. First step tomorrow is to check all the vacuum lines and run some cleaner through the old girl. The truck runs incredibly well, gas mileage is top notch, and I'm somewhat mechanically inclined, but far from a pro. Any help would be greatly appreciated. Thank you.

 

The EGR system controls your NOx numbers. That's what it does. That's all it does!

Well, yes, the Cat has some impact on NOx, and if your O2 sensor is way off you could end up running so lean that the cat is overwhelmed, but usually that causes HC and CO to go nuts.

So I'm 99% sure your problem is somewhere in the EGR system, so that's where I'd start looking first. http://personal.utulsa.edu/~nathan-b...77exhaustg.pdf

One thing to consider with our "older" vehicles is "crud" on the EGR tube that goes into the plenum. The PCV system sprays gunk onto that tube, and if bad enough it can actually block it (and lots of other stuff in the plenum). So your EGR valve and modulator might be working correctly, but nothing is getting into the intake.

(BTW, California (and I think Canadian) vehicles have an EGR gas temperature sensor to catch just this problem. (With no flow into the intake, the temp. won't rise.) Is yours a 'Federal' vehicle?)

 

I took the EGR valve off and checked the lines, they didn't appear to be clogged. Went ahead and installed a new EGR, drove it around for a while, then went in for a retest. These are the new numbers after the new Cat and EGR vlave.

15mph-
HC- 127max 314gp 21meas -pass
CO- .55max 2.08gp 0.01meas -pass
NO- 721max 1822gp 1137meas -fail

25mph-
HC- 75max 264gp 3meas -pass
CO- .69max 2.29gp 0.01meas -pass
NO- 623max 1622gp 1813meas -fail, gross polluter

My numbers are actually getting worse. The only other thing I can think of is the EGR temp sensor, and after that I'm not terribly knowledgable on emissions. My funds are limited to the work I can do myself, and right now I'm stumped.

 

Are the 50 thousand feet of vacuum line routed correctly? Lol

 

They'd better be. I chased them around today and replaced some that looked worn.The truck runs great and I have no problem with gas mileage, there's never been a CEL since I've had it and I keep up on maintainence. It's got new plugs, wires, filters, oil, premium gas, new Cat, new EGR valve, even a new gas cap. I'm running out of ideas. I had a big leaky 95 Suburban with the exhaust cut off at the muffler that I beat to hell, and it never failed smog.

 

Did you actually test the modulator and EGR valve? Replacing parts without testing them can get expensive quickly. The link I provided gives some tests. If the modulator is not working (or if any of the vacuum lines are not working correctly) the EGR won't be do anything.

Since you seem to be "getting worse," a quick acid-test is to put vacuum on the EGR valve actuator at idle; it should produce a noticeable idle change (it might even kill the engine).